This invention relates to a methodology for forming a microfabricated device, wherein the device includes a cavity.
Microelectromechanical systems are devices which are manufactured using lithographic fabrication processes originally developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be made in very small sizes. MEMS techniques have been used to manufacture a wide variety of transducers and actuators, such as accelerometers and electrostatic cantilevers.
MEMS devices are often movable, they may be enclosed in a rigid structure, or device cavity formed between two wafers, so that their small, delicate structures are protected from shock, vibration, contamination or atmospheric conditions. Many such devices also require an evacuated environment for proper functioning, so that these device cavities may need to be hermetically sealed after evacuation. Thus, the device cavity may be formed between two wafers which are bonded using a hermetic adhesive.
One such device which may require evacuation is an electrostatic MEMS switch, wherein a set of conductive terminals is closed when an electrostatic potential is placed between a set of nominally parallel plates. The plates may be located above and below a cavity. The force arising from the application of the electrostatic potential may cause a deflection of the upper plate toward the lower plate, or the lower plate toward the upper. In any event, this motion may be used to make contact between a set of electrical conductors. Various techniques for creating this evacuated cavity are described in, for example, U.S. Pat. No. 7,528,691, issued May 5, 2009, U.S. Pat. No. 7,893,798, issued Feb. 22, 2011 and U.S. Pat. No. 7,864,006, issued Jan. 4, 2011. In each case, the method requires forming a cavity using a hard standoff, and sealing the cavity with another substrate applied to the hard standoffs. Similarly, U.S. Pat. No. 8,698,376 describes a cavity which becomes a buried cavity upon bonding with a piezoelectric substrate. The techniques are not suitable for other architectures which may, for example, require a cavity buried below a substrate surface.
U.S. Pat. No. 8,633,553 issued Jan. 21, 2014 discloses a process for forming a buried cavity within a body of semiconductor material, separated from a top surface of the body by a first surface layer. The cavity is formed with an access duct for fluid communication between the buried cavity and an external environment, and the cavity is etched through this duct. The duct remains with the device, and no method is set forth for sealing this duct. Accordingly, this method does not yield a vacuum cavity required for many MEMS devices.
Accordingly, the formation of sealed vacuum cavities in microfabricated devices remains an unresolved problem.